This invention relates generally to substrates having a hydrophobic layer formed thereon and to products which include such treated substrates, and more particularly to techniques for fabricating products in which the layer incorporates hydrophobic fumed silicon dioxide.
A hydrophobic substance is one having a distinct tendency to repel water in a manner usually characteristic of non-wetted, oily, waxy or fatty materials. A hydrophobic surface will normally not sustain a water slim, even one of monomolecular thickness. This property not only is found in all oils, fats, waxes and many resins, but also in finely-divided powders such as carbon black and magnesium carbonate.
A hydrophilic substance has a strong affinity for water by absorption or adsorption even to the point of gradual liquifaction by extracting water vapor from the atmosphere. This property is characteristic of carbohydrates such as algin, vegetable gums, pectins and starches as well as complex proteins like gelatin and albumen.
The present invention deals with a hydrophobic layer formed on a substrate, which layer incorporates particles of hydrophobic fumed silicon dioxide (HFSD). Silicon dioxide particles are produced by the hydrolysis of silicon tetrachloride in a flame process. The fumed silicon dioxide particle is hydrophilic in nature by reason of the large number of hydroxyl groups present on the surface. These particles are rendered hydrophobic by reacting them with a silane. During the reaction, hydrophobic hydrocarbon groups replace many of the hydroxyl groups, the resulting particles offering increased compatibility with organic or non polar media and a corresponding repulsion to water. One commercially available form of hydrophobic fumed silicon dioxide powder is manufactured and sold by Cabot Corporation of Boston, Massachusetts under the trademark "Silanox".
Silane which is a member of the silicone family, contributes its inherent hydrophobicity and oleophilicity to the HFSD particle. Fumed silicon dioxide, which is a fine pure powder, brings to HFSD a particle of extremely small size and enormous surface area, all of it being accessible for interaction with the surrounding media. In protective coatings, the hydrophobicity derived from the silane component of HFSD is augmented by the surface micro-roughness imparted by the silicon dioxide component, giving rise to a degree of water repellency so great that it is often referred to as super-hydrophobicity.
Substrates coated with HFSD repel water to an extraordinary degree. An air layer becomes entrapped between the substrate and the water and is visible as a reflected silvery sheen. This air layer or shield is sometimes referred to as a gaseous plastron. In order therefore to protect metal against corrosion, one may apply thereto a coating of HFSD which functions to insulate the metal surface from water by means of an air layer. Similary, ice build-up on ship and airplane surfaces may be alleviated by the application thereto of an HFSD coating which causes ice release due to the minimal interfacial contact between the ice and the tiny particles of HFSD.
The super-hydrophobic properties of HFSD can be imparted to substrates in various ways. HFSD particles can, for example, be applied in dry form on tacky surfaces, or it can be applied from a liquid dispersion. But regardless of the mode of applying HFSD to the substrate, it is important that these particles remain essentially uncoated and exposed at the solid-water interface to afford the micro-roughness necessary for optimum super hydrophobicity.
One serious difficulty often encountered in HFSD coatings is its poor abrasion resistance, for if the coating is subjected to wear, it may be eroded, with a consequent loss of super-hydrophobicity and a possible gain in hydrophilic properties should the underlying substrate be hydrophilic in character.
Heretofore, in order to attain a high degree of abrasion resistance, it was the practice to use conventional powder coating methods such as fluid-bed or electrostatic spraying techniques to deposit a blend of dry powder fluids of HFSD and fine resinous powders onto the substrate. Heat is then used to activate the powder resin to allow wetting of the HFSD particles and to thereby bond these particles to the substrate. Typical blends for this purpose make use of as little as 5% to 10% by weight of HFSD in combination with epoxy, vinyl or polypropylene powders to produce coatings having the desired hydrophobic characteristics.
In as much as the binder component in such blends is dominant, the resultant coating is strongly influenced by the characteristics of the resinous binder which is generally much less hydrophobic than the HFSD particles. Moreover, since heat is required to effect bonding, this not only adds to the complexity and cost of the technique but in some instances it may adversely affect the structure of the substrate.